This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Analysis of the molecular identity of the pH sensor in the prokaryotic potassium channel KcsA identified several key mutants that stabilized the KcsA inner bundle gate in the fully open conformation at neutral pH, a fact that makes possible the crystallographic analyses of KcsA in a variety of functional states. As a first approach, we planned to determine the crystal structure of open KcsA via analysis of several of these constitutively open KcsA mutants. Additional mutations in the selectivity filter were designed towards the determination of a fully conductive and an inactivated form of the channel. A preliminary crystal structure of one of these mutants in complex with a Fab has provided tantalizing new information regarding the mechanism of gate opening in KcsA and a likely path towards the fully open state.